[tri-med] Evelyn: Genetics 101 - types of trisomy

> There seems to be some confusion at present about the different types of
> trisomy, how they occur and risk factors.
>
> In general there is a difference in theories as to how "full" trisomy,
> partial trisomy and mosaicism occur. They are not the same and the risks
> are
> not the same. I have pulled out most of my genetics 101, 102 posts so I am
> going to attempt to pull them all together again for the new folk.
>
> For more detailed explanations (with pictures!!) please visit both my
> homepage
> http://homepages.tig.com.au/~karens
> and CDO's web site
> http://members.aol.com/cdousa/intro.htm
>
> First up there are essentially a number of different types of trisomy:
This
> refers to ALL chromosomes even though in general I am using 18 as the
> example.
>
> 1. "full" trisomy (and when people simply refer to trisomy this is what
> they
> should mean - putting "full" in front is actually misleading)
> This is where every cell in the body (as far as they can tell) has the
> additional chromosome.
>
> 2. Partial Trisomy
> now there are two different types of partial trisomy (though this is often
> not distinguished.
> This is where there is part of an extra chromosome in every cell (as far
as
> they can tell). Very basically a chromosome can be divided into two parts
> the p (for petite or short arm) and the q arm (named because scientists
are
> imaginiative and its the next letter in the alphabet!!)
>
> So Trisomy 18p for example refers to there being an extra p arm and not
the
> q. Trisomy 18q refers to there being an extra q arm and not a p.
>
> There is also partial Trisomy p or q. What that means is that there is
part
> of the p or q arm not the whole arm. This is the type of partial that
> occurs
> more frequently.
>
> This sort of trisomy often has some "funny" numbers attached :-) so to
> write
> it "correctly it might be partial trisomy 18 (32.1, ter). meaning that it
> is
> only a "small" extra piece of the chromosome from the band 32.1 to the end
.
>
> 3. Unbalanced translocations.
> This is the other side of balanced translocations. In a balanced
> translocati
> on all the genetic material is there its just not in the right place. This
> can happen in any format. But for example part of the 18th chromosome may
> be
> attached to the 16th chromosome and part of the 16th is where the 18th
> should be.
>
> Because all of the genetic information is there (and provided there are no
> inversions) there shouldn't be a problem with the person. However problems
> to begin when they want to have children. Because of the way the
> chromosomes
> come together and the egg divides it is possible for the baby to get only
> part of the "balanced" translocation. So for example the baby may get the
> part of the "extra" material that is on the 16th chromosome and be missing
> that part of the 16th that is on the mother's 18th.
>
> This is called an unbalanced translocation - because all of the genetic
> material is not there - and in this example would be partial trisomy 18
and
> monosomy 16. That is there is some extra 18th chromosome and some missing
> 16th chromosome. It effects the baby essentially by giving them a mix of
> problems - they can have some of the problems of Trisomy 18 and some of
the
> problems of monosomy 16.......... How much depends exactly where the break
> points occur.
>
> While unbalanced translocations CAN mean a that one of the parents is a
> "carrier" it can also happen "de nova" that is part of a chromosome just
> breaks off and attaches to another chromosome during cell division.
>
> When someone refers to a carrier they USUALLY are referring to balanced
and
> unbalanced translocations. Technically no other type of trisomy (other
than
> a Robertsonian translocation) can have a "carrier".
>
> 4. Robertsonian Translocations
> These are in a totally different category to balanced and unbalanced
> translocations.
>
> A Robertsonian translocation only effects chromosomes 13, 14, 15, 21 and
22
>
> These chromosomes have very short "p" arms. and a Robertsonian
> translocation
> happens when the two "q" arms join together to form one long chromosome.
>
> A balanced translocation happens when part of one chromosome breaks off
and
> attaches itself to another chromosome. With a Robertsonian translocation
> there is no "breakage" (hence no break points) the two complete
chromosomes
> attach themselves together.
>
>
> 5. Mosaicism
> This is where there is a whole extra chromosome in some of the cells and
> some of the cells have the normal compliment of chromosomes. How the child
> is effected depends on how many cells have the extra chromosome AND where
> those cells are (distribution). This is the very hard part of mosaicism
> because NO-ONE can tell ahead of time where those cells lie.
>
> Of course mosaicism can happen in all of the above different types of
> Trisomy (except "full" trisomy) as well. So a person can be mosaic partial
> Trisomy 18p. Rare but can and does happen.
>
> 6. Miscellaneous other :-)
> there are lots of other problems that can effect chromosomes - some have
no
> bearing on Trisomy, some such as inversions, only very rarely. Most of
> these
> types of problems are "new" and they are only just starting to understand
> them because of new technology such as FISH. If I tried to explain them
> this
> post would get even more complicated than it already is!!!!
>
> I hope all of that makes sense.
>
> Take Care and Keep Looking for Rainbows!!!
> Karen, mum to Alex (5, T-18 mosaic)

                  Building ___ooOOoo__ Rainbows
                       www.trisomyonline.org
                  Families Helping Families On-line

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